As is well known, current technologies for making semiconductor integrated circuits have enabled the resistance of the interconnections and contact areas in the active areas of the individual devices to be significantly reduced through the use of composite materials comprising silicon and a transition metal such as titanium or tungsten. These composite materials are called silicides, and are used for producing layers with relatively low resistivities.
The formation of a silicide layer over the active areas of MOS transistors comprises the following steps, once the transistor gate has been formed:
implanting first portions of the source and drain regions with dopant at a low concentration;
forming spacer elements adjacent to the gate and the interconnection lines;
implanting second portions, included in the source and drain regions of the transistor, at a high concentration;
depositing a transition metal over the entire surface of the substrate;
carrying out a thermal process wherein the transition metal will react selectively with the substrate surface to yield the silicide.
These process steps result in the silicide layer being also deposited over the polysilicon which forms the gates and interconnections of the transistor, since the etching steps for clearing the active areas of the oxide which is covering them have a similar effect on interconnections provided by polysilicon lines.
These silicide layers cannot be utilized in the manufacturing of high voltage devices, specifically of HV (High Voltage) transistors either of the P-channel or N-channel type, formed using the DE (Drain Extension) technique. In these devices, the source and drain diffusions are provided as lightly doped regions to obtain HV transistors whose breakdown voltage is set sufficiently high to withstand high bias and working voltages.
It is indeed in these regions that, due to their low dopant concentration and relatively small thickness, the process for making silicide layers may develop problems. For example, in carrying out the thermal process for reacting the transition metal layer with the substrate surface, some of the dopant in the substrate is taken up by the silicide layer at the expense of a substrate surface layer, so that in normal operation the silicide will become shorted to the substrate.